Gypsum-based building boards are widely used as interior materials for building because of their inexpensiveness as well as their excellent characteristics such as fire prevention and fire-resisting performance, sound-insulating performance, heat-insulating performance, and workability. Examples of gypsum-based building boards include gypsum boards, fire-resistant gypsum boards, regular hard gypsum boards, gypsum boards containing glass fiber tissues, and glass mat gypsum boards.
In general, in addition to being uniform in thickness, width and length dimensions and having a predetermined strength, having a smooth surface without any exterior defects that are harmful in use, such as chips, cracks, contaminations, dents, and deflection, is desired of these gypsum-based building boards as an important quality. One of these defects detrimental to quality is air gaps or so-called captured air bubbles captured at the time of forming.
Captured air bubbles refer to air bubbles relatively large in size so as to be easily identifiable by human eyes, which are included in a gypsum core when a gypsum slurry hardens to form a product after passing through a forming machine with air captured inside when a material passes through the forming machine (upper paper, a gypsum slurry, and lower paper pass through the forming machine in the case of a gypsum board, for example) at the time of manufacture of a gypsum-based building board. Usually, captured air bubbles are spherical or ellipsoidal air bubbles (hereinafter referred to as “air gaps”).
When an air gap is generated in a gypsum-based building board product, a depression or bulge is likely to appear on a surface of part of the product in which part the air gap is contained inside the core of the product. This prevents formation of a smooth surface and is thus not preferable in terms of quality.
Furthermore, in terms of quality, there is a problem in that it is not possible to fix the gypsum-based building board product by driving a nail or screw into the part containing the air gap because of the absence of gypsum, which is supposed to form the core.
Therefore, in general, in forming gypsum-based building boards, a pool of gypsum slurry is provided right in front of the entrance of the forming machine and various measures are taken to keep its retained amount constant.
It is difficult, however, to keep the amount of retained gypsum slurry constant because of variations in the quality or the amount of supply of a raw material and differences in the type of a manufactured product and manufacturing conditions. Therefore, there has been no solution so far to the generation of air gaps.
As a result, it is desired to detect and select generated air gaps with reliability. For example, gypsum board cover paper is present at the surface of a gypsum board and gypsum is present at the surface of a gypsum board containing glass fiber tissues. Therefore, the presence of air gaps cannot be visually confirmed unless a dent or bulge appears on the surface of the gypsum-based building board or except in a section of the board.
Therefore, there has been a demand for a method of detecting an air gap inside a gypsum-based building board without cutting or breaking the product.
Patent Document 1 discloses a method of manufacturing a sheet member, in which an optical image of a sheet is first detected and possible defects generated on a surface of or inside the sheet are detected from the optical image. Then, the possible defective parts of the sheet are exposed to infrared radiation or a cooling gas or heating gas is jetted to the possible defective parts of the sheet, and the kinds of defects are determined based on the temperature change characteristics of the possible defective parts in their state of heat generation due to the absorption of infrared radiation or based on the temperature change characteristics or the like of the possible defective parts in their state of heat radiation or the like, thereby grading and manufacturing sheets.
Patent Document 2 discloses a detector and a method for detecting a defective part of a sample, where the detector includes first means for heating or cooling a sample, second means for applying a thermal action reverse to that of the first means to the sample simultaneously with the heating or cooling by the first means, and infrared radiation detecting means for detecting infrared radiation from the sample during the simultaneous heating and cooling of the sample.